The Custom CSS editor was previously powered by Ace. In 2.6, we updated the module and now use another library, named Codemirror. You can find several articles on Google comparing the 2 editors, and both editors obviously have their pros and cons, but we chose to move to Codemirror for several reasons. Here are a few:
- Ace was huge. Codemirror, on the hand, is a lot smaller; the entire library is only 131.7kb, compared to 579.9kb for Ace, or roughly 22.7% the size of the older editor.
- The Ace editor overrode a lot of browser functionality and broke a few of your usual browser keyboard shortcuts. Codemirror is better about not breaking normal browser behaviour.
- Implementing Codemirror allowed us to fix several outstanding bugs with the old editor. Syntax highlighting is now better, tab management is also better, among other things.
- The new editor is better integrated with WordPress Core.
I hope this clarifies things a bit.
You can find a summary of the changes in the following changesets:
https://plugins.trac.www.ads-software.com/changeset/795348
https://plugins.trac.www.ads-software.com/changeset/795350
https://plugins.trac.www.ads-software.com/changeset/802710